Human DNA topoisomerase 1 (TOP1) has been established as an effective new molecular target for anticancer drugs. These agents inhibit (poison) TOP1 by trapping the covalent enzyme-DNA intermediate (termed the cleavable complex), thereby stimulating protein-linked DNA strand breaks, which represent a new form of DNA damage that effectively kills tumor cells. Terbenzimidazoles (TBs) represent a new structural class of TOP1 poisoning agents that bind in the minor groove of DNA. Select TB derivatives exhibit potent TOP1 poisoning activities and cytotoxicities exceeding that of the clinical camptothecin derivative, topotecan. However, the specific molecular interactions that govern the TOP1 poisoning and cytotoxic activities of the TBs are still poorly understood. The overall goal of this proposal is to develop the TBs as effective anticancer agents. Toward this goal, we will employ a broad range of computational, biophysical, biochemical, and cellular techniques to achieve the following specific aims: (i) Determine the TOP1-targeting specificity as well as the TOP1-mediated cytotoxicity of TB derivatives. These studies are designed to evaluate the potencies of the TBs at trapping TOP1-DNA cleavable complexes in intact cells as well as to establish their TOP1-directed cytotoxicities. (ii) Define the specific drug-enzyme and drug-DNA interactions that stabilize the ternary TB-DNA-TOP1 cleavable complex. We will use computational techniques in conjunction with available crystallographic information to generate explicitly solvated structural models of ternary TB-DNA-TOP1 cleavable complexes, and experimentally assess the validities and predictive integrities of our generated models. (iii) Enhance the TOP1 poisoning and cytotoxic efficacies of the TBs by reducing their propensities for self-association. In addition to the development of compounds with increased efficacy, these structure-activity studies will also result in compounds with enhanced water solubilities, a feature that has enormous potential value with regard to the ease of drug delivery. (iv) Determine the role of DNA minor groove binding in the poisoning of TOP1 by TBs. These studies are designed to enhance our understanding of the molecular mechanism by which the TBs poison TOP1. The information gleaned from our proposed studies will enable us to develop a rational approach to the design and development of next generation TB compounds that exhibit predictably enhanced TOP1 poisoning and cytotoxic activities.